This invention relates to a process control system for a Claus sulfur plant. More specifically, this invention relates to a temperature control strategy for the initial reaction zone of a Claus thermal reactor having at least two reaction zones, wherein the relative concentrations of sulfur-containing reactants to molecular oxygen-containing reactants in the first reaction zone are varied.
Some art relevant to sulfur plants is discussed briefly hereinafter.
U.S. Pat. No. 3,970,743 (1976) to David K. Beavon discloses an improvement to the Claus process for sulfur manufacture which enables continuous operation even with hydrogen sulfide feeds which contain substantial quantities of fixed nitrogen compounds, e.g. ammonia. The inability of conventional Claus plants to tolerate more than 0.5 mole percent ammonia in feed gases is mentioned. Also mentioned are the difficulties from plugging that arises while processing hydrogen sulfide-fixed nitrogen compound feed streams, e.g., H.sub.2 S-NH.sub.3 mixtures. Generally, in order to decompose all ammonia, high temperatures are to be preferred, and in order to decrease the concentration of sulfur trioxide produced, higher temperatures are also to be preferred. And, finally, the optimum conditions to run the reaction of hydrogen sulfide to sulfur dioxide are such that there is a deficiency of oxygen present, i.e. an "oxygen-lean" condition. The specific improvement disclosed involves a thermal reactor having first and second zones in series. The first reaction zone contains air, fixed nitrogen compounds and hydrogen sulfide. The reaction product from the first zone is then mixed with additional hydrogen sulfide in the second reaction zone. The desired stoichiometry of the Claus reaction in terms of the amounts of oxygen and hydrogen sulfide necessary is discussed.
Not discussed in U.S. Pat. No. 3,970,743 (1976), however, is the desirability and the strategy to control temperature in the first zone of the thermal reactor. Good temperature control in the first zone of the thermal reactor is very important in order: to avoid damage to the unit from runaway thermal conditions, to maximize ammonia decomposition, and to minimize the amount of sulfur trioxide produced during oxidation in the first zone of a thermal reactor.
U.S. Pat. No. 3,985,864 (1976) of Lucien H. Vautrain, et al., and Re. 28,864 (1976) of Andval, et al., each disclose an automated control system for a Claus sulfur plant. The flow rate of the oxygen-containing gas to a process for the oxidation of hydrogen sulfide in U.S. Pat. No. 3,985,864 is regulated so as to be responsive to changes in pressure in the hydrogen sulfide feedstream. In both patents, the overall ratios of oxygen to hydrogen sulfide are adjusted to maintain the desired ratio of hydrogen sulfide to oxygen feed. In carrying out stoichiometric control of the hydrogen sulfide gas stream and oxygen-containing gas stream, there are five objectives cited in U.S. Pat. No. 3,985,864. These objectives are (1) maintain the quantity of oxygen below that stoichiometrically required for the oxidation of the hydrogen sulfide in order to prevent the formation of sulfates; (2) maintain the oxygen quantity as close as possible to the stoichiometry required in order to promote the highest possible efficiency of oxidizing the hydrogen sulfide-containing gas stream and to reduce the sulfur content of the gaseous effluent from the process; (3) maintain stable control of the process while achieving the above two objectives, even though the gas flow may vary; (4) maintain stable control, even though the hydrogen sulfide content of the hydrogen sulfide gas-containing stream may vary; and (5) effect stable control of the process while achieving the above four objectives, even though there is a time between the occurrence of a variation in one or both of the process feedstreams and the occurrence of the measurement of the affect of that variation on the gaseous effluent from the process. In summary, both patents disclose an automated flow control scheme to maintain the required stoichiometry of the Claus reaction.
A Claus sulfur plant throughout this specification is intended to mean a plant for converting hydrogen sulfide alone or in combination with fixed nitrogen, e.g. ammonia, to produce sulfur. U.S. Pat. No. 3,970,743 (1976) of D. K. Beavon, which is expressly incorporated herein in its entirety, along with references cited therein, discloses a process for the production of sulfur from mixtures of hydrogen sulfide and fixed nitrogen compounds. While this patent discloses the desirability of running the initial reaction zone of a Claus thermal reactor in an oxygen-lean state, there is no temperature control strategy to optimize overall performance of such a reactor.
Attempts to control temperature in the first zone by an automatic method for varying the fraction of predominantly hydrogen sulfide-containing gas introduced into the first zone which only follows temperature will likely have problems and fail to control temperature properly. This is likely because the same temperature in the first zone can be obtained at two different relative amounts of sulfur-containing reactants. For example, if the first zone were in an "oxygen-rich" condition, then a decrease in sulfur-containing reactants will lead to a decrease in temperature in the first reaction zone; but, if the first zone were in an oxygen-lean condition, then the decrease in sulfur-containing reactants will lead to an increase in temperature in the first reaction zone. In summary, the relative concentrations of the sulfur-containing reactants to molecular oxygen-containing reactants must be known before a "simple automated instruction" to a control valve controlling the flow of the sulfur-containing reactants can be defined. By "simple automated instruction" is meant a command to open or close a particular valve by some incrementally desired amount.
It is, therefore, a principal object of this invention to provide a unique temperature control strategy and method for carrying out the Claus reaction which optimizes overall performance of a Claus sulfur plant.
It is an object of this invention to provide a unique method for controlling the temperature in the first of two interconnected thermal reaction zones of a Claus thermal reactor, wherein the first zone is more upstream than the second.
Other objects of this invention are clear based upon this specification.